Switched-mode power supplies are known in the art. These devices can be used to transform an alternating signal from a mains network, e.g. 230V at 50 Hz, into a DC or AC signal with different amplitude. The switched-mode power supply of the present invention is mainly directed towards transforming an alternating electrical supply signal into a DC signal, wherein the DC signal is used to feed a load.
A switched-mode power supply typically comprises a rectifier which is arranged to convert an electrical supply signal provided at its input, e.g. from a mains network, into a rectified electrical supply signal emerging at its output. It also contains an energy storage for storing electrical energy. This energy storage can be connected to the load to be fed. In this storage, electric field energy and or magnetic field energy, or electromagnetic energy in general, can be stored.
Furthermore, the supply comprises a controllable switching element that is connected to the energy storage. The switching element is arranged to switch the switched-mode power supply between a charging state, in which the energy storage is charged by the rectifier via charge transport from the rectifier, and a discharging state, in which the energy storage releases at least part of electrical energy stored therein to the load. Charge transport, such as current flow, results in build-up of energy in the energy storage. For instance, current through an inductor gives rise to a magnetic field. The energy associated with this field can be regained if the inductor is subjected to a negative current change.
A switching element controller is used for controlling the switching element. It is arranged to control the switching element to switch the switched-mode power supply from the charging state to the discharging state when the charge transport exceeds a current limit.
During operation, the switched-mode power supply alternates between the charging state and discharging state at a switching frequency which is substantially higher than a frequency of the rectified electrical supply signal. As such, in a period of the rectified electrical supply signal, the switched-mode power supply changes states multiple times. Typically, the switching frequency of the supply is in the order of 10 kHz-10 MHz, compared to a frequency of roughly 100-120 Hz for the rectified supply signal.
Recently, LED-based lighting solutions have emerged on the market that can compete with the traditional incandescent or fluorescent lighting. LED-based lighting is more energy efficient and has a longer service life than traditional lighting products.
LED-based lighting typically comprises a plurality of LEDs which are controlled by a LED driver. Most often, the LED driver itself comprises a switched-mode power supply. Especially for domestic lighting solutions, the LED driver can normally be connected to the mains, e.g. 110V or 230V, of the home electricity network.
A typical circuit configuration of a known LED-based lighting device is schematically illustrated in FIG. 1A. The device comprises a LED driver 1 driving a plurality of LEDs, LED-1 . . . LED-X. The driver comprises a rectifier 2 which performs a full-wave rectification on an alternating electrical supply signal at its input terminals 3, 3′, e.g 230V @ 50 Hz. The rectified signal is fed to LED-1 . . . LED-X through switching element 4, which causes a time-varying current through LED-1 . . . LED-X. The frequency of the time-varying LED current, which corresponds to the switching frequency, is substantially higher than the frequency of the rectified electrical supply signal.
The switched-mode operation of the LEDs improves the power efficiency compared to the situation in which the LEDs are connected to the rectifier directly albeit using a resistor connected in series to limit the current.
A problem with these known LED-based lighting devices, or LED drivers, is the low power factor and high harmonic distortion that is commonly associated with highly non-linear devices.
A low power factor indicates that a significant amount of the power delivered to the LED lighting system is reactive. Reactive power presents a challenge for the energy suppliers in that it does not contribute to the power dissipated/used by the consumer but it does increase the load on the electricity network due to high currents and/or voltages which can occur in them.
Another problem related to non-linear electronic devices is harmonic distortion which introduces unwanted high frequency components in the network. These signals could interfere with other components connected to the network.
Typical I-V characteristics of a known LED-based lighting device are illustrated in FIG. 1B. As shown, the current is only drawn from the mains network when the applied voltage is high. In addition, the current waveform is non-sinusoidal and out of phase compared to the applied voltage. As a result of these factors, the power factor is low and the harmonic distortion high.
It is therefore an object to provide a switched-mode power supply capable of operating with a high power factor in which the above mentioned disadvantages do not occur, or at least in a lesser degree.